Since melamine resin has active groups as active hydrogen, active methylol, active alkoxymethyl or the like, it is customarily combined with a number of coating base resins having functional groups which are reactive with the abovesaid active groups, like hydroxyl and isocyanate, including alkyd resin, polyester resin, acrylic resin, epoxy resin, polyurethane resin, polyamide resin and polycarbonate resin, and is widely used as thermosetting curable coating compositions. However, in such coating compositions based on the combination of melamine resin and the abovementioned base resins, it is generally required to use a relatively high baking temperature as, for example, 140.degree. C. for the combination of commercially available melamine resin and alkyd resin, and 170.degree. C. and more for the combination of such melamine resin and epoxy resin. From the standpoint of energy saving, much preference is given to the use of lower baking temperature. It is, however, pointed out that with the abovementioned melamine resin, a lower temperature baking will inevitably cause insufficient coating hardness and undesired tack feeling on the coating and therefore, it has been believed that the curing will require a minimum baking condition of 140.degree..about.160.degree. C. and 20.about.30 minutes under the circumstances.
Recently, there has been provided, as the so-called low temperature curing type resin, a highly self-condensable melamine resin, which is said to be curable at 100.degree..about.120.degree. C. in 20 to 30 minutes. However, even with this type of melamine resin, there are such problems that the allowable temperature range in obtaining the baked coating with defined film performance is rather narrow (i.e. considerable temperature dependency for baking temperatures) and if the baking temperature control is not so good, Tg and crosslinked density of the coating will fluctuate in wider ranges and the intercoat adhesion will get worse due to the change in coating shrinkage and accumulation of inner stress of the coating. Therefore, in the combination with a variety of base resins, a melamine resin has long been desired which will show a small temperature dependency for baking temperatures, is curable at a relatively lower baking temperature and is excellent in inter-coat adhesion. The invention has been made to satisfy this need.